1. Field of the Invention
The present invention relates to a technology of radio communication and in particular, relates to a radio communication technology in which a radio communication device having a plurality of antennas communicates with a radio communication device having one antenna.
2. Description of the Related Art
In recent years, a high-speed radio communication device installing a MIMO (Multiple-Input Multiple-Output) technique which can dramatically increase transmission capacity appeared. In the MIMO technique, a plurality of transmitting and receiving antennas are utilized. Each antenna transmits a different signal (data). Thus, the MIMO technique performs parallel transmission through transmitting a plurality of data streams. Therefore, the high-speed radio communication device installing the MIMO technique can perform multiplex communication by the parallel transmission. As a result, the high-speed radio communication device installing the MIMO technique can increase transmission capacity. At this moment, a small number of the high-speed radio communication devices installing the MIMO technique are utilized in a market of the radio communication device. The MIMO technique is compatible with a technique by which communication is achieved in a form of one data stream (IEEE802.11a, IEEE802.11g and IEEE802.11b). Therefore, the high-speed radio communication devices installing the MIMO technique and radio communication devices which communicate in the form of one data stream interoperate in a radio communication system (For example, Japanese Patent Application Laid-Open No. 2006-014027).
FIG. 1 is a system configuration diagram according to a related art. In FIG. 1, a radio communication device which communicates in a form of one data stream and a high-speed radio communication device installing a MIMO technique are interoperate.
With reference to FIG. 1, a wireless LAN device 10 includes three antennas for transmitting and receiving. The wireless LAN device 10 can transmit and receive data in a form of three data streams via the three antennas. That is, the wireless LAN device 10 can communicate according to the MIMO technique. Antennas 11-1 to 11-3 are transmitting and receiving antennas which are mounted in the wireless LAN device 10.
A wireless LAN device 12 transmits and receives data in the form of one data stream. That is, the wireless LAN device 12 communicates in the form of one data stream without the MIMO method. An antenna 13 is a transmitting and receiving antenna which is mounted in the wireless LAN device 12.
In a configuration mentioned above, the wireless LAN device 10 communicates in the form of one data stream via one predetermined antenna of the three antennas. Meanwhile, other antennas are not used for communication in the form of one data stream. In FIG. 1, the wireless LAN device 10 communicates with the wireless LAN device 12 in the form of one data stream. That is, the antennas 11-1 and the antenna 13 are used for communication. According to the communication mentioned above, antennas 11-2 and 11-3 are not used for communication at all and consequently, they become useless. Moreover, the antennas 11-2 and 11-3, which are useless for communication, cause a change of radio transmission characteristic of the antenna 11-1 due to the antennas 11-2 and 11-3 being arranged near the antenna 11-1. And the change of the radio transmission characteristic of antenna 11-1 causes bad influence on the radio transmission between the antenna 11-1 of the wireless LAN device 10 and the antenna 13 of the wireless LAN device 12.